Here is a link to a thread with some tips already posted
I hope it helps you
The fact it was painted over shouldn't change it too much. It will just make a bit tougher. but some wall papers have almost a painted somewhat sealed Finnish to begin with.
With the painted surface it may be even more important to score it. Their are hand tool that you run over the surface to perforate the outer layer to allow the moisture top get threw to loosen it up.
Link to example of as scorer tool